Fast stochastic codebook search through the use of odd-symmetric crosscorrelation basis vectors

A fast codebook search method for code excited linear predictive (CELP) coding of speech is described. The method relies on using a vector-sum codebook where the crosscorrelation of any pair of basis vectors is odd-symmetric. Due to this odd-symmetric crosscorrelation (OSC) property the energy term of the cost function for codebook search is a constant with respect to the search, and a simple sign detection procedure is used to locate the optimum codeword with a complexity almost independent of the codebook size. An algorithm for generating a set of OSC basis vectors is described. Simulation results show that by replacing the standard VSELP codebooks with the OSC codebooks, the same coder performance can be achieved but with a much lower complexity. An OSC-CELP coder was implemented and demonstrated to achieve good quality speech at rates below 4.8 kbps.